I think there's a bit of camo around those B-pillars to make them look wider than they actually are. Take a look at the driver's side window and it appears there's a black strip that isn't part of the interior paneling.

Accounting for that, the central B-pillar looks to be the same width as the F30, and I suspect the rest of it is camo to hide the fact that the 4GC will have frameless glass, a la the 6GC and other BMW coupes.